Lightning Bolt Physics
Nathaniel McClain II
PHY 3091
Florida State University
Lightning Bolt Physics

Benjamin Franklin, more
than 200 years ago, proved
that lightning was an
electrical discharge and
measured the sign of the
cloud charge that produced
it. Modern research on the
physics of lightning began in
the early 20th century with
the work of C.T.R. Wilson,
the same scientist who
received the Nobel Prize for
his invention of the cloud
chamber.
Lightning Bolt Physics

Most research on the electrical
structure of clouds has focused
on the cumulonimbus, the
familiar thundercloud or
thunderstorm, because this
cloud type produces most of the
lightning. There have been
limited studies of the electrical
properties of other types of
clouds such as stratus,
stratocumulus, cumulus,
nimbostratus, altocumulus,
altostratus, and cirrus clouds
that might potentially produce
lightning.

The classic model for the
charge structure of a
thundercloud was developed
in the 1920's and 1930's
from ground-based
measurements of both
thundercloud electric fields
and the electric field
changes that are caused
when lightning occurs.
Lightning Bolt Physics

In this model, the thundercloud forms a
positive electric dipole as shown in Figure
1.1 and Figure 1.2; that is, a primary
positive charge region is found above a
primary negative charge region. By the
end of the 1930's, this overall structure
had been verified from measurements
made with sounding balloons inside
clouds and had also identified a small
localized region of positive charge at the
base of the cloud. Subsequent
measurements of electric fields both
inside and outside the cloud have
confirmed the general validity of this
double-dipole structure. However, in any
given cloud the charge distribution can be
more complex, and there is often a
negative screening layer above the
primary positive charge region.

Lightning is a transient, high-current
discharge whose path length is
measured in kilometers. Well over
half of all flashes occur wholly within
the cloud and are called intracloud
(IC) discharges. Cloud-to-ground
(CG) lightning has been studied
more extensively than other forms
of lightning because of its practical
importance (for instance, as the
cause of injuries and death,
disturbances in power and
communication systems, and the
ignition of forest fires) and because
lightning in the clear air below the
cloud base is more easily studied
with optical techniques.

Cloud-to-cloud and cloud-to-air
discharges occur less frequently
than either IC or CG lightning.
All discharges other than CG
are often combined under the
general term cloud discharges.
Lightning Bolt Physics

A negative CG discharge (Figure 1.1,
category 1) begins in the cloud and
effectively lowers some tens of Coulombs
of negative charge to Earth. The total
discharge is termed a flash (as is the total
discharge for other types of lightning).
Flash durations are typically about half a
second. A flash has several components,
the most significant being three or four
high-current pulses called strokes. Each
stroke lasts about a millisecond, and the
separation between strokes is typically
several tens of milliseconds. Lightning
often appears to "flicker" because the
human eye can just resolve the individual
pulses of luminosity that are produced by
each stroke.
Lightning Bolt Physics

When the return-stroke current
ceases, the flash, including
various discharge processes
within the cloud, may end. In
that case, the lightning is called
a single-stroke flash. On the
other hand, if additional cloud
charge is available, a
continuous dart leader can
propagate down the residual
first-stroke channel and initiate
another return stroke.

During the time between the end of
the first return stroke and the
initiation of a dart leader, so-called
J- and K-processes occur in the
cloud. The J-process involves
charge motion in the cloud on a
tens-of-milliseconds time scale,
while the K-process moves charge
on a time scale ten times shorter.
Lightning Bolt Physics

The time between
successive strokes in a
flash is usually several
tens of milliseconds, but
can be tenths of a
second if a continuing
current persists in the
channel after a return
stroke.
The End